true for americium-241 solubility. This increase in plutonium solubility also occurred during the earlier studies of Barth (1975) and Barth and Mullen (1974). The data of Barth and Mullen (1974) indicated that the immediate increase in plutonium solubility during the jejunal stage was due specifically Although bilary excretion of americium-241 has been to the presence of bile. suggested by Scott (1948) (as reported by Durbin, 1973), indicating that bile has the ability to complex americium-241, there was no consistent increase in the solubility of americium-241 during the jejunal incubation period. In making comparisons of radionuclide concentrations in rumen contents between trials, only very large differences are meaningful since the cattle may graze at will throughout the enclosures. Also, the plant type ingested and the resuspension of radionuclides may vary. However, in the available data, a trend concerning peak activity of plutonium in the rumen contents appears to be developing. Results to date (Tables 2 or 3 of Barth, 1975, and Tables 3 or 4 of this report) indicate that the highest concentrations of plutonium in the rumen contents of cattle grazing at Area 13 have occurred during the late summer or fall, followed by reduced concentrations during the winter and early summer. An exception to this occurred during trial 11. Unfortunately, observations were not made of the field conditions of the Eurotia lanata during these trials. However, visual observations were made of the field conditions of Furotia lanata at Area 13 during the summer and fall of 1975 and the winter of 1975/1976 (data not included in this report). The reproductive phase, characterized by the long-haired fruiting involucres, appeared during October, 1975. During January, 1976, some fruiting involucres remained. The onset of the reproductive phase will vary depending on rainfall and other conditions. This suggests that with the presence of involucres, the entrapment of resuspended particles would be greatly increased, resulting in higher concentrations of plutonium and americium-241 in the rumen contents. This plant is described, with a photograph of the reproductive phase, by Wallace and Romney (1972). The moderate increases in plutonium and americium-241 solubility during trials 7 and 10, compared to other trials which used rumen contents collected from cattle grazing in the inner enclosure, were associated with a reduction in the intake of Eurotia lanata and an increase in the intake of Atriplex conferttfolta. Barth and Mullen (1974) reported average minimum and maximum solubilities of 1.5% and 8.4% for plutonium-238 dioxide in an artificial rumen and simulated abomasal and intestinal fluids procedure. However, during trial 9 of the present study, solubilities of over 62% for both plutonium-238 and plutonium-239 were shown, while in the earlier report of Barth (1975), the solubility reached 90% and above during spring and midsummer trials. This suggests the possibility that during these specific trials, most of the ingested plutonium was in a form other than plutonium dioxide and that most of this plutonium represented the low level internally incorporated in plant tissue. Primary factors which may contribute to the marked increase in plutonium and americium-241 solubility during trial 9 (outer enclosure) are a greater distance 132 ye